In a computer system including a memory device, the timing of signals and operations of devices external to the memory device are controlled by a system clock, while the timing of signals and operations of devices inside the memory device are controlled by an internal clock. In order to ensure integrity of signals transmitted between the memory device and external devices, the internal clock is required to be synchronized with the system clock. Usually, the memory device includes a delay locked loop (DLL) circuit which provides the internal clock based on the system clock.